Abstract
Higher-dimensional topological phases play a key role in understanding the lower-dimensional topological phases and the related topological responses through a dimensional reduction procedure. In this work, we present a Dirac-type model of four-dimensional topological insulator (TI) protected by symmetry, whose 3D boundary supports an odd number of Dirac cones. A specific perturbation splits each bulk massive Dirac cone into two valleys separated in energy-momentum space with opposite second Chern numbers, in which the 3D boundary modes become a nodal sphere or a Weyl semimetallic phase. By introducing the electromagnetic (EM) and pseudo-EM fields, exotic topological responses of our 4D system are revealed, which are found to be described by the mixed Chern-Simons theories in the low-energy regime. Notably, several topological phase transitions occur from a -broken TI to a TI when the bulk gap closes by giving rise to exotic double-nodal-line or nodal-hyper-torus gapless phases. Finally, we propose to probe experimentally these topological effects in cold atoms.
- Received 7 April 2022
- Revised 29 September 2022
- Accepted 19 October 2022
DOI:https://doi.org/10.1103/PhysRevLett.129.196602
© 2022 American Physical Society